Field of the Invention
Embodiments of the present invention generally relate to a method and an apparatus for controlling fluctuations in power and amount of power used at an electric load location and/or on an electrical grid.
Description of the Related Art
Energy demand at a commercial site, such as a business, or at home will vary over the time of day. In a typical home or hotel setting, there is a peak in the morning when the occupants get up and when the occupants return home at the end of the day. This typically creates two demand peaks during a normal day. Commercial buildings tend to follow different patterns depending on the nature of the business. For example, usage is typically low when a commercial building is closed, and may be relatively constant or fluctuate between moderate to high levels depending on the type of business when the building is open. For example, a car wash may have more fluctuations in its energy use than an office building in a moderate climate.
The cost to a utility for generating or purchasing electrical energy increases dramatically during periods of peak use versus periods of off-peak usage. In order to compensate for the higher peak-hours costs, utility companies often employ time of day-based rate schedules, charging a significantly higher rate for electrical energy (e.g., cost per kilowatt-hour (kW-hr)) consumed during peak usage hours as compared to energy consumed during off-peak hours. For example, homes and businesses may pay for electricity on a per-kilowatt hour basis with one rate applying during off-peak hours, and another, higher, rate applying during peak hours. The higher rates charged during peak usage periods can lead to significantly higher energy costs for the user, especially when the user's period(s) of high demand coincides with or falls within the interval set by the utility as peak hours.
Devices have been developed that help users reduce the cost of electricity purchases from the power grid by storing electricity in energy storage mediums, such as batteries, that can be “drawn down” during peak hours to reduce demand from the grid. The batteries can be charged during non-peak hours, thus reducing the total cost of electricity, and, during favorable conditions, electricity can even be sold back to the grid. This process is often referred to as “energy arbitrage,” which is generally the storing of energy at one time of day and then the discharging of energy at another time, effectively shifting energy consumption from one time period to another.
Energy storage mediums, especially battery energy storage, are expensive and, while various techniques are known by which an energy storage system can be used to optimize energy use at a business or home, such techniques are generally inefficient in applying stored energy to effectively control the energy use at an electric load location. Consequently, an impractical quantity of energy storage mediums are required at an electric load location to realize useful energy arbitrage. Generally, such energy storage systems use simple methods for controlling the charging and discharging of power provided to offset peak demands. For example, two approaches commonly used in energy storage systems include: Using a simple timer to control charge times of the energy storage system (typically during off-peak hours) and discharge times (typically during peak demand hours); and using a single demand set-point that the storage system reacts to while monitoring and controlling the energy use of the business or home location. A single demand set-point generally is a single level set-point to which the controlling element in an energy storage system will control during operation. Each of these approaches generally requires an uneconomical amount of energy storage capacity in order to offset demand peaks at the electric load location. Furthermore, use of a single demand set-point typically results in an energy storage system running out of energy storage availability due to over-reaction of the controlling components to the demand set point set by the controlling system. Thus, a need exists for power charge and discharge systems and methods that more effectively utilize the consumable energy storage medium components in an energy storage system.
Today's electrical power grid in the United States consists primarily of large synchronous power generators, such as hydroelectric generating facilities and natural gas combustion turbines. In the United States, these types of power generators generate electricity to meet demand at a frequency of 60 Hz. If a source of electricity from a large power generator (e.g., power plant) is dramatically reduced, system frequency and the speed of other system interconnected generators, which are electrically connected to the grid, will decrease. To compensate for the shift in frequency, the other conventional power generators, at one or more generating facilities that are attached to the grid, automatically respond via their governor control schemes by creating more power to increase the system generators' speed and frequency, attempting to bring system frequency back to 60 Hz. Additionally, in an oversupply situation, for example if a wind farm unexpectedly increases production through a large wind deviation over its scheduled production level, system frequency increases and the other synchronous generators that are connected to the grid reduce their production to automatically bring the system frequency back down closer to 60 Hz. The stability of the electrical power grid has been changing with an increase in the number of wind and solar generators that are not equipped with automatic governor controls and are displacing the quantity of power delivered by traditional synchronous generators. Not only is wind and solar generation unpredictable, but these generators also have no automatic control schemes to help maintain system synchronous speed and no event response capability during their process of delivering power to the grid. This creates a significant and growing need for automatic frequency balancing grid-connected devices to maintain 60 Hz. These devices must not only be able to balance frequency events frequently and repeatedly, but also act very quickly to help stabilize the system frequency as the radiation of the sun and the speed of the wind change renewable production levels constantly and near instantaneously. Therefore, there is a need for a method and apparatus that is able to help minimize, reduce and/or prevent and correct for fluctuations in the frequency and/or power being transmitted across local and larger electric interconnected grids.
Also, an electric grid exists to move electric power from energy sources, such as power generators, to energy sinks, such as power users. Distribution grids move this power over short distances, while transmission grids allow for electricity transmission over long distances. During grid events, such as when there is electricity supply disruption, due to transmission or generation failure or renewable energy source(s) variability, electricity generation from the power generators can become out of step, or sync. The lack of synchronization can both be due to the time it takes for power to travel over long distances and the different speeds at which generators respond to meet these demand and supply changes. Therefore, the relative power angle (e.g., angular difference in the phasor angles of voltages at different points along a transmission system) becomes out of sync across geographically disparate areas, and power oscillations occur. The power oscillations create swings in the amount of power cycling back and forth on the transmission and distribution lines that can both damage transmission and distribution equipment and create additional energy losses in the power lines, thereby de-rating the grid's transmission capacity.
FIG. 20 illustrates a graph of the amount of power flowing through a portion of the electric grid as a function of time (i.e., curve 2001). As illustrated in FIG. 20, the graph illustrates two different grid events and their effect on the power flowing through the portion of the electric grid. In this example, a first grid event (e.g., Event 1) is experienced by the grid at a time of about 200 seconds. While oscillation in the power flow in the grid does occur after the first grid event, the amount of power fluctuation is not significant enough to cause large fluctuations in the power flowing in the electric grid. However, one will note that the second grid event (e.g., Event 2) causes a much larger drop in power flowing through the electric grid which causes larger undamped oscillations to occur in the electric grid, as the power generator attached to the electric grid try to correct for the drop in power transmission through the grid. Typically, these power oscillations cannot be measured at a single node on the grid, and thus require multiple measurement nodes to determine the oscillation.
It is believed that the undesirable effect of having the relative power angles from two or more power generators being out of sync becomes a larger issue when moderate to large power fluctuations occur within the grid (e.g., oversupply or undersupply situations caused by a grid event). California's transmission grid has several transmission lines with constant power oscillations, some of which cause the transmission operator to place significant restrictions on transmission line capacity (e.g., restrictions on the power delivery). This, in turn, restricts the import of clean and inexpensive hydropower from the Pacific Northwest, for example.
As an example, if Los Angeles experiences a large load in the late afternoon of a hot day and a nearby power plant or transmission line fails, then there is a sudden undersupply scenario in the power distribution and transmission grid in Southern California. The local power generators in Southern California “see” a new electricity demand, due to failure of the power plant or transmission line, as do distant power generators in the Northwest. One will note that the Northwest power generators are coupled to the Southern California power grid by use of various transmission lines that form part of the power grid in California. Then both the local and Northwest power generators respond to meet the increased power demand created by the power plant or line failure, but the generators in the Northwest are generally relatively slower to respond, due to electrical transmission delays and the nature of the power generating devices used to supply power to the grid. The difference in the timing of the delivery of power from the local and Northwest power generators then creates an oversupply situation in Southern California, as both power generators try to compensate for the variation in the power demand. Then, to compensate for the oversupply situation, the local generators reduce their amount of power generation, as do the distant Northwest power generators. Therefore, again, an undersupply situation occurs as both generators try to compensate for the over-supply situation. This sequence occurs over and over again as the power flowing through the transmission lines in the California grid repeatedly oscillates back and forth, due to this generator cycling. The relative power angle between these two generators increases and decreases as the oscillations occur.
Greater occurrences of these power swings and instability in the delivery of power through the various grid distribution and transmission lines have been happening more frequently across the US. The North American Synchrophasor Initiative (NASPI) is currently building a monitoring system to detect and analyze system oscillations across the US power grid to better understand the affect of the displacement of a large number and quantity of conventional synchronous generation with variable renewable supply sources, such as wind and solar power generators. There is a belief that the addition of renewable energy source will cause the electric grid to become more unstable as more conventional synchronous generation are displaced by the variable power generation created by most renewable energy sources.
Therefore, there is a need for a system that can help damp or minimize the oscillations found in at least a portion of the electrical power grid that occur due to normal or abnormal power generation or power transmission events. There is also a need for fast-acting resource(s) that can counter and eliminate system instability and power swings, which are created by the addition of renewable sources intermittently delivering power to the grid.